1. Field of the Invention
The present invention relates to a color cathode ray tube equipped with an in-line electron gun so constituted as to emit three electron beams in one horizontal line toward a fluorescent screen.
2. Description of the Prior Art
In a cathode ray tube equipped with at least an electron gun comprising a cathode and a plurality of grid electrodes, a deflection device, and a fluorescent screen, the following arts have been known to obtain a preferable reproduced image extending from the central portion to the periphery of the fluorescent screen: one for providing an astigmatic lens in a region of an electrode constituting a focusing lens (main lens), and the other for forming an electron beam passing hole of the main lens constituting electrode of an in-line electron gun into a slot and making the sizes of central and side electron beam passing holes different (Japanese Pat. Laid-Open No. 64368/1976).
This type of color cathode ray tube, as shown in FIG. 1, is equipped with at least a vacuum vessel comprising a panel 61, a funnel 62, and a neck 63 which are made of an insulator such as glass, an electron gun 64, a shadow mask 65, and a fluorescent screen 66 contained in the vacuum vessel, and reproduces an image by impinging electron beams emitted from an electron gun 64 onto the fluorescent screen 66.
FIG. 2 is a sectional view of an essential portion of a main lens, schematically illustrating the structure of a conventional in-line electron gun used for the above cathode ray tube.
In FIG. 2, reference numerals 08, 09 and 010 are cathodes, 011 is a first grid electrode, 012 is a second grid electrode, 013 is a third grid electrode which is one of the electrodes constituting a main lens, 014 is a fourth grid electrode which is the other electrode constituting the main lens, 015, 016, and 017 are inner cylinders connected to the opening portions of the third grid electrode 013 on the fourth grid electrode 014 side, and 018, 019, and 020 are inner cylinders connected to the opening portions of the fourth grid electrode 014 on the third grid electrode 013 side. Numerals 021, 022, and 023 are central axes of electron beams, respectively and the central axis 022 of the center electron beam is aligned with the axis of the electron gun (tube axis). These central axes 021, 022, and 023 are aligned with the openings corresponding to the cathodes 08, 09, and 010 of the first, second, and third grid electrodes 011, 012, and 013, and with the central axes of the inner cylinders 015, 016, and 017 connected with the opening portions of the third grid electrode 013, and they are arranged on the same plane almost in parallel.
The central axes of the central opening portion of the fourth grid electrode 014 and the inner cylinder 019 connected to the central opening portion are aligned with the central axes 022. However, the central axes of the opening portions on the both sides and the inner cylinders 018 and 020 connected to the opening portions are not aligned with their corresponding central axes of the third grid electrodes, but they are slightly shifted outwards.
Symbol S in FIG. 2 represents the interval between central axes 021, 022 and 023 of the electron beams, L represents the distance between the central axes 021 and 023 of the outer electron beams and the inner wall of the neck, and D represents the inside diameter of the inner cylinder connected to the opening portion of the G3 electrode 013.
The in-line electron gun having the above constitution operates as shown below.
Thermionic electrons emitted from three cathodes 08, 09, and 010 heated by a heater are attracted toward the first grid electrode 011 by a positive voltage applied to the second grid electrode 012, and three electron beams are formed. Then, these three electron beams pass through the openings of the first grid electrode 011 and then through the opening of the second grid electrode 012. The beams are accelerated by positive voltages applied to the third grid electrode 013 and the fourth grid electrode 014, and enters the main lens.
In this case, a low voltage of approximately 5 to 10 kV is applied to the third grid electrode 013 constituting the main lens; a high voltage of approximately 20 to 35 kV to be applied to the fluorescent screen is applied to the fourth grid electrode 014 through a conductive film coated on the inner wall of the funnel 62. Therefore, a electrostatic field is formed between the third grid electrode 013 and fourth grid electrode 014 by the difference in voltage between the third grid electrode 013 to which the low voltage is applied and the fourth grid electrode 014 to which the high voltage is applied. Therefore, the paths of three electron beams in the main lens are bent by the electrostatic field. As a result, three electron beams are focused on the fluorescent screen.
Moreover, because the central axes of the opposing openings of cylinders for side beams of the third grid electrode 013 and fourth grid electrode 014 are not aligned with each other, the main lens for the side beams is not symmetric about the central axis. Therefore, the side electron beams are so deflected inward that they are converged in accordance with the center electron beam on the fluorescent screen. Thereby, three electron beams are converged on the fluorescent screen, images of three colors of R, G, and B generated by three electron beams are correctly registered, and a color image is displayed.
In an in-line electron gun constituted as described above, three electron beams do not satisfy the convergence conditions due to slight variations of the electron gun component accuracy and assembling accuracy. Therefore, it is necessary to make adjustment for convergence of electron beams.
In this convergence adjustment, as the beam spacing S between the electron beams decreases, deviation of the electron beams from the convergence conditions decreases and the adjustment gets easier. From past experiment results, it has been known that it is preferable to set the S value to less than approximately 5 mm.
In conventional focusing electrode structures, however, the opening diameter of the focusing electrode is restricted to a value smaller than the beam spacing S between the adjacent electron beams entering the lens. Therefore, a limit is put on the opening diameter for setting the beam spacing S between electron beams to be less than 5 mm.
The effective aperture of the focusing lens of each electron beam is determined by this opening diameter. Therefore, a problem arises that the spherical aberration of a lens increases and the electron beam spot diameter increases as the opening diameter decreases.
To solve the above problem, a structure is known which is disclosed in Japanese Pat. Laid-Open No. 103752/1983. This structure makes it possible to decrease the spherical aberration while the beam spacing S is maintained at less than 5 mm.
The structure of the electron gun disclosed in the above publication will be schematically described below, referring to FIGS. 3(a) and 3(b). FIG. 3(a) is a longitudinal sectional view of the essential portion, illustrating the main lens of an in-line electron gun and FIG. 3(b) is a transverse sectional view of the essential portion of FIG. 3(a), taken along the line Axe2x80x94Axe2x80x2 of FIG. 3(a).
In FIGS. 3(a) and 3(b), reference numeral 13 is a third cylindrical grid electrode whose opening cross section is almost elliptic, 14 is a fourth cylindrical grid electrode whose opening cross section is also almost elliptic, 13-1 is a flat electrode provided in a third grid electrode 1, 14-1 is a flat electrode provided in a fourth grid electrode 2, 13R, 13G, and 13B are electron beam passing holes (openings) of the flat electrode 13-1, 14R, 14G, and 14B are electron beam passing holes (openings) of the flat electrode 14-1, and 21, 22, and 23 are central axes.
As shown in FIG. 3(b), the diameter D in the direction (vertical direction) perpendicular to the in-line direction (horizontal direction) of the openings 13R, 13G, and 13B of the flat electrode 13-1 of the third grid electrode 1 is approximately equal to the diameter of the main lens formed by the electrode. As the diameter D increases, the spherical aberration decreases and also the electron beam spot diameter decreases.
However, even in the above structure, another problem described below arises.
That is, to increase the vertical direction diameter D and to decrease the electron beam spot diameter at the fluorescent screen, it is necessary to increase the electron beam diameter in the main lens electrode. In this case, if the vertical direction diameter D is extremely larger than the beam spacing S of adjacent electron beams, a problem is caused that electron beams strike a flat electrode in the grid electrode, especially when the beams are of large currents.
It is an object of the present invention to provide a cathode ray tube equipped with an in-line electron gun causing no problem in convergence of three electron beams and allowing the main lens diameter to increase in such a way that the electron beams do not strike the flat electrode in the third grid electrode.
To achieve the above object, the present invention provides a color cathode ray tube equipped with an in-line electron gun comprising at least electron beam producing means for emitting three electron beams of in-line arrangement toward a fluorescent screen and main lens means for focusing the three electron beams on the fluorescent screen, being provided with a flat electrode having electron beam passing areas in two cylindrical electrodes which are arranged at an interval in the direction of the travel of the electron beams emitted from the electron beam producing means and have approximately-elliptic opening cross sections kept at different potentials, characterized in that when the distance between the centers of three adjacent electron beams is denoted by S (mm), the opening diameter of two cylindrical electrodes perpendicular to the in-line electron beam arrangement direction is denoted by D (mm), the above S and D meet the following relations:
S less than 5.00,
D greater than S, 
and
55 Sxe2x88x9220Dxe2x89xa7145.5
Moreover, the color cathode ray tube is characterized in that each of the mutually facing openings of the two cylindrical electrodes constituting the main lens means comprise a single opening for the three electron beams.
Furthermore, the color cathode ray tube equipped with an in-line electron gun constituted as described above may involve a problem that, if the distance between electron beams and the inner wall of the neck for housing the in-line electron gun is too small, the inner wall of the neck comes to a high potential due to the high voltage applied to the funnel portion of the color cathode ray tube, the electron beams are deflected due to an electric field produced by the high potential of the inner wall of the neck glass, and three electron beams are not converged on the fluorescent screen, when the color cathode ray tube is continuously operated for a long time.
To increase the distance between electron beams and the inner wall of the neck for housing the in-line electron gun, it is necessary to increase the neck diameter or decrease the beam spacing S of the adjacent electron beams.
However, if the neck diameter is increased, the funnel diameter also increases, the distance between the electron beams and the deflection yoke increases, and the deflection sensitivity of the deflection yoke is degraded.
If the beam spacing S is decreased, a problem is brought up that the distances decrease between the beams and the electrodes of the main lens separating the electron beams from each other in the main lens where the diameters of the electron beams are largest, and the electron beams strike the main lens electrode.
If the electron beam diameter in the main lens electrode is decreased to avoid the strike, a problem arises that the electron beam spot diameter on the fluorescent screen increases because the lens magnification decreases and the space charge effect increases. Moreover, if the beam spacing S is decreased, another problem arises that the spherical aberration of the main lens increases and the electron beam spot diameter on the fluorescent screen is further increased because the lens aperture D must be also decreased when the main lens is made up of the electrodes each having three circular openings as shown in FIG. 2.
It is another object of the present invention to provide a color cathode ray tube equipped with an in-line electron gun in which the above problems of the prior art are solved and the focus characteristic is improved by eliminating the influence of the potential of the neck inner wall and decreasing the static convergence drift under a long-time operation.
To achieve the above object, according to the present invention, a color cathode ray tube equipped with an in-line electron gun having electron beam generation means for emitting three electron beams toward a fluorescent screen and a main lens comprising two electrodes kept at different potentials and provided separately from each other in order to focus the three electron beams on the fluorescent screen, characterized in that when the outside diameter of the neck 63 (FIG. 1) for housing the in-line electron gun is denoted by T (mm), the beam spacings between the central axes of adjacent electron beams are denoted by S (mm), the above T and S meet the relations, 2 S+14.6xe2x89xa6Txe2x89xa625.3, and the beam spacing S is 4.1 mm or more.